A wind-tunnel investigation of the wind speed and turbulence characteristics close to the ground over various escarpment shapes

A wind-tunnel investigation of the wind flow over two-dimensional forward-facing escarpments is reported as part of a continuing research programme into the effects of local topography on the wind flow close to the ground. Four sharp-edged escarpments with their slopes varying between a cliff and a 4:1 gradient, were placed normal to a simulated neutrally-stable rural boundary layer which was modelled to a scale of 1:300. The resulting flows close to the surfaces of the escarpments were measured with a hot-wire anemometer. The modifications to the mean wind speed, turbulence intensity and energy spectra over the escarpments are described. The results indicate the extent and magnitude of the modification to the flow and suggest that significant changes in turbulence characteristics only occur in the wake region close behind the crest, where a shift of energy to higher frequencies is evident.     

On convective turbulence and the influence of rotation

A series of experiments were performed in a rotating cylindrical tank over a wide range of rotation rates in which convective turbulence was generated by a bottom-mounted heated plate in both homogeneous and stratified fluids. Measurements were made of the turbulent velocities in all three axes over the full depth of the chamber, and of the temperatures at the mid-depth near the centre of the tank. For even small rotation rates, the measurements showed that the turbulent velocities were weakly affected by rotation at all depths, but as the rotation rate increased, the deviation from the non-rotational scaling slowly and progressively increased until eventually the turbulent velocities were fully rotationally controlled. The results indicated that there was no sudden transition of the turbulent field from the non-rotational state (a function only of the surface buoyancy flux B and the depth z) to the rotational state (where the strength of the turbulent field is a function of only B and the Coriolis parameter f). Rather the transition was a smooth asymptotic one from one state to the other. Nevertheless, it was possible to parametrize this transition by a single value of the turbulent or small scale Rossby number, defined by Ro = (B/f³z²)^1/3. Our measurements suggested a critical value of Ro_c ≈ 0.1, below which the turbulence was fully rotationally controlled and which was equivalent to a critical depth z_c = (35 ± 15)(B/f³)^1/2. Using typical oceanic values for B and f, the oceanic turbulence driven by surface cooling events becomes rotationally controlled only for depths greater than about 10 km, a depth which is greater than that of the bulk of the world's oceans. Thus, convective turbulence actively being generated by cooling of the ocean surface is best described by non-rotating turbulent velocity and length scales and is a function only of the surface buoyancy flux and the depth.     

On the Nonlinear Stability of Three-Dimensional Quasigeostrophic Motions in Spherical Geometry

Nonlinear Mobility criteria for the motions geoverned by three-dimensional quasigeostrophic model in spherical geometry are obtained by using Arnol’d’s variational principle and a priori estimate method. The results gained in this paper are parallel to Arnol’d’s second theorem and better than the known results. Especially, under the approxima-tion of vertically integrated nondivergency, criteria corresponding to Arnol’d’s second theorem are first established by a detailed analysis.     

Some Effects of Rotation Rate on Planetary-Scale Wave Flows

A series of experiments were performed in a rotating annulus of fluid to study effects of rotation rate on planeta-ry-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth’s atmosphere, although magnitude of variation in earth’s rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number deter-mining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with the variation in earth’s rotation rale. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.     

Symmetric Stability of Rotation and Boussinesq Fluid in Bounded Domain by Using Normal Mode Method

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Some effects of rotation rate on planetary — scale wave flows

Summary A series of experiments was performed in a rotating annulus of fluid to study effects of rotation rate on planetary-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary Waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth's atmosphere, although magnitude of variation in earth's rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number determining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with variation in earth's rotation rate. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.With 12 Figures     

Wind rotation characteristics of the upper tropospheric monsoon over the central and eastern tropical Pacific

在本文中,为研究赤道中东太平洋区域上对流层季风是否具有和经典季风同样的风向变化特征,我们引入一种风向角的研究方法——有向转角法,该方法不仅可以刻画风向变化的角度大小,而且包含风矢量如何转变的信息。在此基础上,利用该方法并将其适当推广,本文研究了赤道中东太平洋区域上对流层季风的演变过程,结果证实了该区域上对流层季风具有和经典季风同样的四种转变特征,并进一步给出了随季节转换不同时期内的风向演变结果,发现上对流层季风相对经典季风具有爆发时间较早并结束较晚的特点。本文结果是对全球季风系统理论的有益补充,进一步支持该理论将全球不同季风作为一个整体系统来看待。     

Observed spatiotemporal characteristics of drought on various time scales over the Czech Republic

This paper analyses the observed spatiotemporal characteristics of drought in the Czech Republic during the growing season (April to September) as quantified using the Standardised Precipitation Evapotranspiration Index (SPEI) on various time scales. The SPEI was calculated for various lags (1, 3, 6, 12, and 24 months) from monthly records of mean temperature and precipitation totals using a dense network of 184 climatological stations for the period 1961–2010. The characteristics of drought were analysed in terms of the temporal evolution of the SPEI, the frequency distribution and duration of drought at the country level, and for three regions delimited by station altitude. The driest and the wettest years during the growing season were identified. The frequency distribution of the SPEI values for seven drought category classes (in per cent) indicates that normal moisture conditions represent approximately 65 % of the total SPEI values for all time scales in all three regions, whereas moderate drought and moderate wet conditions are almost equally distributed around 10.5 %. Differences in extremely dry conditions (5 %) compared with extremely wet conditions (1.5 %) were observed with increasing SPEI time scales. The results of the non-parametric Mann–Kendall trend test applied to the SPEI series indicate prevailing negative trends (drought) at the majority of the stations. The percentage of stations displaying a significant negative trend for the 90, 95, 99, and 99.9 % confidence levels is approximately 40 %. An Empirical Orthogonal Functions (EOF) analysis was used to identify the principal patterns of variability of the SPEI during the growing season that accounted for the highest amount of statistical variance. The variance explained by the leading EOF range 66 to 56 %, whereas for EOF2 and EOF3, the value is between 7 and 11 % and between 4 and 7 %, respectively, for the SPEI is calculated for 1- to 24-month lags.     

Weather effects on the aerosol characteristics in the surface layer of the atmosphere

On the basis of 14-year (1994–2007) series of the semi-annual (January to June) measurements of the atmospheric aerosol microstructure in Dolgoprudny, Moscow region, effects are analyzed of pressure, wind speed, precipitation, and surface temperature inversions on the aerosol particle concentration. It is shown that the weather parameters affect concentration of the particles within a certain range of sizes (0.1 to 1 μm). Concentration of the smaller particles practically does not depend on the weather conditions. The weather effects on large (> 1 μm) particle concentration are not detected due to too high variability of the latter. From three the most available weather parameters (pressure, wind, daily precipitation amount), the observed weather conditions are classified, and for the detected types of weather, typical concentrations are determined of the aerosol particles of different sizes, which allow approximately estimating, from the standard weather data, the aerosol pollution under different meteorological situations.     

The effects of land-surface characteristics on the East Asian summer monsoon

 The Community Climate Model version 2 (CCM2) of the National Center for Atmospheric Research (NCAR) was used to investigate the effects of the land-surface characteristics on the East Asian summer monsoon. Four numerical experiments were performed in this study. They include the control run, the biosphere–atmosphere transfer scheme (BATS) run, the heavy snow run, and the light snow run. The results show that CCM2 can reasonably simulate many characteristics of the East Asian summer monsoon, such as the 850-hPa southwesterlies, 200-hPa easterlies, high precipitation rate, two monsoon subsystems, the low-level subtropical high, and the upper level South Asian anticyclone. Nevertheless, the model still exhibits some systematic errors, including oversimulation of the temperature over the Eurasian continent, which in turn intensifies the monsoon circulations. In the BATS run, the model can significantly relieve the temperature bias over the continent in spring and early summer. However, the effect of BATS decreases in the summer due to excessive incoming solar radiation. The Eurasian continent is still occupied by an oversimulated thermal low in summer. In the heavy snow case, the high albedo of snow and larger soil moisture suppress the warming rate of the surface and atmosphere in the early summer and hence the cooler troposphere results in a weaker monsoon circulation. Moreover, anomalous cyclonic flows are found in the leeside of Tibetan Plateau (i.e. the southwest vortex in China) in the heavy snow case. This may shed a light on the precipitation anomalies (floods) over Yangtze River Valley (Central China) and eastern Asia due to intensified baroclinic disturbances. Received: 8 September 1999 / Accepted: 5 June 2000     

高大建筑物影响城市粗糙副层流场特征的数值研究

用数值模拟方法研究了高大建筑物对城市粗糙副层气流场特征的影响。数值模式采用基于雷诺平均纳维—斯托克斯方程组的应用计算流体力学FLUENT软件,次网格湍流参数化选用k-ε闭合方案。建筑物用立方体表示,并规则排列于模拟区域内。通过改变高大建筑物的数量与位置,对建筑物阵列内及其上空的气流特征进行了多个算例的数值模拟。依据模拟结果计算获得建筑物区域的面积平均风速廓线,结果表明各算例的粗糙副层风速廓线各不相同。运用动力学方法由风速廓线计算出各算例的零平面位移高度和粗糙度,并与几种计算零平面位移高度和粗糙度的形态学方法进行了比较检验。结果表明两种形态学方法(Ba、Ma)计算所得的零平面位移高度与动力学方法计算结果很接近,但对于粗糙度而言,几种形态学方法的计算结果都明显偏高。     

On some characteristics of insolation changes in the past and the future

Variations in terrestrial insolation, induced by perturbations of the earth's orbital parameters, are calculated for different geographical latitudes for ±100000 yr and in detail for the modern period between A.D. 1800–2100. The calculations show that short-period insolation variations occur against a background of secular variation, with an amplitude which can be comparable in magnitude to that of the 300-yr secular trend. For comparison we calculate the secular trends of insolation for Milankovich's caloric half-years for the period ±100000 yr with high time resolution. The nature of secular and short-term insolation changes is discussed for different latitudinal circles during future centuries. We conclude that orbitally-induced variations of insolation with periods of 18.6, 11.9, 5.9, 4.0, and 2.7 yr will perturb the radiation regime at the upper atmospheric boundary.     

起伏条件对冰雹增长的影响

观测资料表明,云中的一些物理量如气流、含水量等都是有起伏的。在雷暴和冰雹云中,由于强烈的扰动,这些量的的起伏更为显著。中国科学院大气物理所的云物理工作者曾经在云滴浓度(或含水量)、上升气流、湍流加速度、电量等气象因子有起伏的条件下,研究了暖云降水机制问题。本文在文献[3]和[5]的工作基础上,将生长质点的半径作为多元随机变量(初始粒子半径和诸气象因子)的函数来处理,对雹胚为多谱时在两个主要气象因子—含水量和湍流加速度有起伏的环境中生长的冰雹谱作了具体的推导和计算。另外还推导计算了其它几种较简单的模式,并作了比较。通过上述工作,揭示了那种平均物理量比较弱的对流云中能够在较短时间里形成中等以上大小(或中等强度)的冰雹的物理原因。定量地说明了起伏的气象因子的增减和雹胚的大小和分布对冰雹谱的影响。     

Numerical simulation of the scavenging rates of ice crystals of various microphysical characteristics

Numerical models of trajectories of small aerosol spheres relative to oblate spheroids were used to determine ice crystal scavenging efficiencies. The models included the effects of aerodynamic flow about the ice particle, gravity, aerosol particle inertia and drag and electrostatic effects. Two electric configurations of the ice particle were investi-gated in detail. The first applied a net charge to the ice particle, of magnitude equal to the mean thunderstorm charge distribution, while the second applied a charge distribution, with no net charge, to the ice particle to model the electric multipole charge distribution. The results show that growing ice crystals with electric multipoles are better scavengers than single ice crystals with net thunderstorm charges, especially in the Greenfield gap (0.1 to 1.0 μm), and that larger single crystals are better scavengers than smaller single crystals. The results also show that the low density ice crystals are more effective scavengers with net charges than they are with charge distribution.     

On the impact of cloudiness on the characteristics of nocturnal downslope flows

The effects of cloud cover amount and the height of cloud base on nighttime thermally induced downslope flow were investigated using analytical and numerical model approaches. The conclusions obtained with the analytical and the numerical model evaluations agreed. It was concluded that, (i) as cloud cover increases and/or the height of cloud base decreases, the depth and the intensity of nighttime thermally-induced downslope flows may decrease by a factor reaching one sixth and one tenth, respectively, in the case of overcast low cloud; (ii) when skies suddenly cloud over around midnight, the development of the downslope flow is altered in different ways: a reduction in intensity; or a cessation of further development, depending on the fraction of cloud coverage, and (iii) with a sudden clearing of overcast low cloud around midnight, the depth and the intensity of the downslope flow increases significantly.On leave from the Institute of Atmospheric Physics, Academia Sinica, Beijing, China.On leave from CSIRO, Division of Atmospheric Research, Private Bag No. 1, Mordialloc, Victoria 3195, Australia.     

On the effects of vertical eddy viscosity on rossby and eady waves in the ocean

The effects of vertical eddy viscosity on simple mesoscale waves in the ocean are studied. The decay of Rossby waves is investigated by one-dimensional depth-dependent linear stability problems which are derived for the interior non-viscous or viscous quasigeostrophic flow using parameterizations of the top and bottom boundary layers corresponding to Ekman suction, no-stress and bottom-stress boundary conditions.The non-slip condition at the bottom yielding an O(E_v^1/2)-Ekman layer causes very short damping times for the 0th Rossby mode. This suggests that this boundary condition is not suitable for mesoscale wave studies, because a Rossby wave fit for the MODE eddy can be done satisfactorily without any damping. Reasonable results for damping times of Rossby waves are obtained by prescribing the bottom stress, resulting from the constant-stress layer at the bottom, and the free-slip condition at the surface. The growth rates of Eady waves are reexamined using this bottom-stress condition.Vertical viscosity in the interior of the ocean, e.g. internal wave induced viscosity, may have a significant influence on the dynamics of the mesoscale motions, comparable to that of the boundary layers in some cases. The results are compatible with the sparse observations available.     

On frictional and orographic effects on precipitation in coastal areas

Methods have been designed to calculate orographic and frictional influences on precipitation in coastal regions. A model computes the vertical fluxes of water vapor at the top of the planetary boundary layer due to these two effects. The horizontal downwind displacement of falling precipitation is also allowed for. The procedures are applied over southwestern British Columbia to monthly amounts and maximum 24-h rainfalls for return periods of 2–50 yr. Correlation coefficients between observed and computed precipitation vary from 0.65 to 0.97, the average being 0.84.     

On the failure of Laplace's tidal equations to model subinertial motions at a discontinuity in depth

Infinitesmal amplitude, inviscid, subinertial oscillations over a discontinuity in depth are considered distinguishing three ocean models: (i) the Laplacian model in which the flow is governed by Laplace's tidal equations (LTE); (ii) the more realistic geophysical model in which the Vaisala frequency is assumed to be much greater than the inertial frequency; and (iii) the laboratory model in which the fluid is homogeneous with the Vaisala frequency equal to zero. The Laplacian model supports free waves perfectly trapped at the step while the laboratory model supports no perfectly trapped waves. The two approximations nevertheless predict similar behavior, because in the presence of forcing, the surface mode of the laboratory model is highly excited at frequencies and wavenumbers close to those of the perfectly trapped solutions predicted by LTE. Only in the limit of very long, low-frequency motions does the Laplacian model describe the barotropic modes of the geophysical model well, qualitatively, and even here the quantitative disagreement in predicted phase speeds and group velocities is substantial. At shorter wavelengths, LTE qualitatively misrepresent the dispersion properties by erroneously predicting topographically trapped motions with vanishing group velocity at a subinertial upper limit to the frequency of free oscillation. In particular, the results indicate that, for the step topography, there is in general no trapped, barotropic mode (double-Kelvin wave) in the geophysical model. Thus, LTE fail in modeling even the barotropic parts of subinertial motions at a depth discontinuity and should not be used in such calculations.     

On the bulk parameterization of surface fluxes for various conditions and parameter ranges

A comparison between various parameterizations for the bulk transfercoefficient for heat and momentum is carried out for a wide range ofatmospheric stability and values of the roughness lengths for momentum and heat,z_0m and z_0h respectively.It is confirmed that the parameterization of Launiainen compares wellto a numerical iterative solution for the Obukhov length L as function of the gradients of wind speed and temperatureover a limited range of z_0m/z_0h and stability conditions.For _0m/z_0h > 500, an alternativeinterpolation formulation of Holtslag and Ek,in combination with the formulation of Launiainen, provides a better approximation.     

On the effects of irrigation and urbanisation on the annual range of monthly-mean temperatures

Summary ?Weather station data indicate that large-scale irrigation of an arid region in southeastern Australia decreased the annual range of monthly-mean temperatures by 1–2 K. Such decrease is consistent with an observed increase in dewpoint, by about 1 K. Urbanisation may reduce the annual range but generally increases it. Received August 23, 2001; revised March 6, 2002; accepted April 14, 2002